Why is sodium reabsorption important for maintaining blood pressure?
It decreases water retention
It increases water reabsorption, expanding blood volume
It reduces cardiac output
It lowers vascular resistance
The Correct Answer is B
A. It decreases water retention: Sodium reabsorption has the opposite effect, as sodium is the primary osmotic determinant of extracellular fluid volume. Decreasing retention would lead to fluid loss and hypotension. Physiological blood pressure maintenance relies on the conservation of sodium to hold water.
B. It increases water reabsorption, expanding blood volume: Active transport of sodium creates an osmotic gradient that draws water from the tubular fluid back into the peritubular capillaries. This expansion of the plasma volume increases venous return and cardiac output. This mechanism is a primary driver of systemic arterial pressure.
C. It reduces cardiac output: Increased blood volume from sodium and water retention actually enhances cardiac output through the Frank-Starling mechanism. A reduction in cardiac output would be a consequence of sodium depletion or heart failure. Sodium conservation supports the circulatory demand for adequate perfusion.
D. It lowers vascular resistance: Sodium retention is often associated with the activation of the renin-angiotensin-aldosterone system, which increases vascular resistance. While sodium itself is an ion, the fluid volume it retains increases the pressure against vessel walls. It does not act as a systemic vasodilator.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is C
Explanation
A. It increases water reabsorption by directly stimulating ADH release: ADH release is triggered by hypothalamic osmoreceptors sensing sodium concentrations and blood volume. Urea itself is not the primary physiological trigger for the neurohypophyseal release of ADH. It is a metabolic byproduct that aids the osmotic gradient.
B. It promotes active sodium reabsorption in the distal tubule: Urea transport is largely passive or facilitated and does not drive the active transport of sodium. Sodium reabsorption in the distal tubule is primarily regulated by aldosterone. Urea's role is restricted to the osmotic environment of the inner medulla.
C. It contributes to the medullary osmotic gradient through recycling: Urea is reabsorbed from the inner medullary collecting ducts and moves into the thin limbs of the loop of Henle. This recycling traps urea in the medulla, accounting for nearly 50% of its hypertonicity. This high osmolarity facilitates maximal water reabsorption.
D. It reduces the permeability of the collecting ducts to water: Urea does not alter the intrinsic water permeability of the ductal epithelium. Water permeability is strictly controlled by the presence of ADH-induced aquaporins. Urea provides the osmotic "pull" that makes that permeability effective for water recovery.
Correct Answer is A
Explanation
A. It increases renal clearance by allowing more excretion: Renal clearance is a measure of how effectively the kidneys remove a substance from the plasma. If less of a filtered substance is reabsorbed into the blood, more remains in the urine. Consequently, the clearance rate increases.
B. It has no direct impact on renal clearance: Clearance is mathematically defined by the ratio of urinary excretion to plasma concentration. Since reabsorption directly determines how much of the filtered load is returned to circulation, any change in reabsorption significantly alters the clearance.
C. It increases tubular secretion: Reabsorption and secretion are distinct physiological processes involving different transport proteins. A decrease in the movement of substances out of the tubule does not inherently trigger an increase in the movement of substances into the tubule. They are independent variables.
D. It decreases renal clearance by reducing excretion: Reducing the reabsorption of a solute prevents it from returning to the peritubular capillaries. This leads to a higher concentration of the solute in the final urine. Therefore, excretion is enhanced, which increases the calculated renal clearance.
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